Process for the production of hydrogen fluoride



Aprll 30, 1963 s. FLEMMERT 7,

PRQCESS FOR THE PRODUCTION OF HYDROGEN FLUORIDE Filed Sept. 12, 1957 F-ond Si-Contoining Row Material I &

Preparation of HFond SiFq n Decomposition SiF to SiO HF III Reaction ofHF and Mixture of z No HF No si f-g 2 11 Decompose to DecOmpQseDecompose to NOF HF 350C. NOF s Decompose at 550' to TOO'C.

F' 5.?! MR.

oos'rA LENNART FLEMMERT his ATTORNEYS United States Patent 3,087,787PROCESS FOR THE PRODUCTION OF HYDROGEN FLUORIDE Giista LennartFlernrnert, Nynasvagen 1A, Nynashamn, Sweden Filed Sept. 12, 1957, Ser.No- 683,444 12 Claims. (Cl. 23-153) The present invention relates to aprocess for the production of hydrogen fluoride, employing cheap andreadily accessible fluorineand silicon-containing raw materials, andmore particularly, to a process for the production of hydrogen fluoridewhich yields silicon dioxide in finely divided form as a usefulbyproduct.

Hydrogen fluoride usually is prepared by the reaction of phosphate rockfluorspar or like impure calcium fluoride containing siliceousimpurities, including silicon dioxide and silicates, with sulfuric acid.The reactions which take place are as follows:

Silicon tetrafluoride is formed as a byproduct by Reaction 2 and thisformation leads to a considerable loss of hydrogen fluoride when thecontent of siliceous impurities is as low as 5%.

This loss of fluorine can be avoided by using a calcium fluoride havinga low silicon content, but this calcium fluoride is an expensive rawmaterial. Therefore, the art has directed itself to the problem ofobtaining as much hydrogen fluoride as possible from thesilicon-containing calcium fluoride raw materials.

Engelson et al. in US. Patent No. 2,631,083 propose to overcome thisproblem by preparing a calcium fluoride having a low content of silicondioxide from the impure calcium fluoride starting material. Engelson etal. point out that for commercial use siliceous fluorspar cannot containmore than about 12% silica. When used as a raw material for theproduction of hydrofluoric acid, the fluorspar cannot contain less than97% calcium fluoride, nor more than 1.5% silica. However, a largeproportion of the siliceous fluorspar deposits in the world fall belowthis minimum standard of purity.

Engelson et al., therefore, address themselves to a method of removingthe silica from the calcium fluoride and thus upgrading it for use inthe production of hydrogen fluoride, to the socalled acid gradecontaining 97% calcium fluoride, or higher. The starting material isreacted with hydrofluoric acid in aqueous solution. The fluosilici-cacid thereby obtained is vaporized and burned, resulting in theformation of hydrogen fluoride and silicon dioxide, and the hydrogenfluoride is used for leaching out silicon dioxide from the calciumfluoride in aqueous solution. The purified calcium fluoride then can beused in the reaction with sulfuric acid to produce hydrogen fluoride.

It has long been known that siliceous fluorspar can be stripped of itssilica content by treating it with hydrofluoric acid, obtaining ahydrofluosilicic acid solution. However, this process is notpracticable, since an equivalent amount of hydrofluoric acid isconsumed, and no net gain of fluorine is achieved. Winter et al. No.2,588,786 propose to avoid this by adding sodium fluoride to thehydrofluosilicic acid solution obtained by extraction of the fluorsparwith aqueous hydrofluoric acid solution. This results in the formationof sodium fluosilicate as a precipitate, and an aqueous solution ofhydrofluoric acid. The hydrofluoric acid solution is separated,concentrated, and treated with a further amount of sodium fluoride,which results in the formation of a precipitate of sodium 3,087,787Patented Apr. 30, 1963 ice bifluoride. Sodium bifluoride is separatedand decomposed thermally at about 500 F. (approximately 260 C.) with theformation of anhydrous hydrogen fluoride. Sodium fluoride which is addedin the process is recovered by thermal decomposition of sodiumfluosilicate, and the sodium bifluoride.

In the reaction of fluorspar and sulfuric acid, silicon tetrafiuoridewhich is formed as a byproduct escapes as a gas. If this material couldbe recovered, the process could be made more practicable. Breton et al.No. 2,410,043 propose to recover this waste silicon tetrafluoride byabsorbing the waste gases in lime, with the formation of calciumfluosilicate. This is decomposed by heat to yield calcium fluoride andsilicon tetrafluoride. The silicon tetrafluoride can be recycled,whereas the calcium fluoride, which is obtained in a pure state, may beused in the production of hydrogen fluoride.

It is apparent that the processes which have been described heretoforehave been directed either to purifying the fluorspar, or to utilizingthe hydrofluoric acid solution extracts which are obtainable from thefluorspar. The purification type processes are disadvantageous in thatthe purification is an additional and costly step, and still leaves theproduction of hydrogen fluoride to the final reaction with sulfuric acidand heat. The extraction type processes have the difliculty that percycle only a small portion of the fluorine in the fluorspar is recoveredin the form of hydrofluoric acid.

In accordance with the process of the instant invention, hydrogenfluoride can be produced directly from impure siliconandfluorine-containing material by producing silicon tetrafluoridetherefrom, and utilizing the silicon tetrafluorid-e as a source ofhydrogen fluoride. Hydrogen fluoride obtained with the silicontetrafluoride from the material is carried through the processunchanged, and recovered at the conclusion with hydrogen fluorideproduced from silicon tetrafluoride. The process is carried out in fourstages, in sequence.

Stage I: The fluorine and silicon-containing raw material is convertedto silicon tetrafluoride and hydrogen fluoride by any method.

Stage II: The mixed hydrogen fluoride and silicon tetrafluoride arereacted with water in the vapor phase to produce silicon dioxide andhydrogen fluoride, and the silicon dioxide which is suspended in thehydrogen fluoridecontaining gas is separated from the gas.

Stage III: The hydrogen fluoride and unreaeted silicon tetrafluoride areabsorbed on solid sodium fluoride, or are condensed to form a mixture ofhydrofluoric acid and hydrofluosilicic acid, which is then treated withsolid sodium fluoride. In either case, sodium bifluoride and sodiumfluosilicate are obtained.

Stage IV: The sodium bifluoride and sodium fluosilicate are decomposedto liberate hydrogen fluoride and silicon tetrafluoride, respectively.

The following reactions take place:

Stage I: (1) H 08F: 03804 l- 2HFT (2) 4HF SlO: SiF4T 21130 Stage II: (3)SiF 2Hg0 $102.1 4H]? Stage III.- (41 HF NaF NaIIF;

(5) SiF, +2NaF NsgSiFl A Stage IV: (6) NaHF, HFT NaF The silicontetrafluoride can be recycled to (3), and the sodium fluoride recycledto (4) and (5). Thus, none of the fluorine in the raw material iswasted; all of it is recovered as hydrogen fluoride.

If a mixture of sodium fluosilicate and sodium bifluoride is obtained,they can be decomposed in sequence,

so as to yield the gases separately in a manner in which they can beseparately recovered. When the mixture is heated to about 350 C. thesodium bifluoride only is decomposed, to yield sodium fluoride andhydrogen fluoride, which escapes and may be recovered by condensation.Next, the temperature is raised to from 550 to 700 C. and air is passedthrough the furnace, whereby silicon tetrafluoride is liberated, and canbe recycled for reuse.

On the other hand, the sodium fluosilicate can be obtained separately byadding just enough sodium fluoride to the initial mixture ofhydrofluoric and hydrofluosilicic acids to form sodium fluosilicate.This is filtered off, and then more sodium fluoride is added in order toprecipitate the hydrofluoric acid in the form of sodium bifluoride. Theseparate salts can then be decomposed when desired by heating to 350 C.and to from 550 to 700 C., respectively.

The process of the invention is illustrated in the drawing which is tobe considered together with the following description.

It will be evident that the process can be operated as a cyclic processin which the only raw materials are the siliconand fluorinecontainingmaterials, acid, and a flame or water for decomposition of the silicontetrafluoride. Hydrogen fluoride is obtained in a pure state at theconclusion of the process, and a finely-divided silicon dioxide is avaluable byproduct.

Under suitable conditions it is possible to convent 95% or more of thefluorine content of the raw materials into hydrogen fluoride. This isbecause substantially none of the fluorine is lost in byproducts whichare not capable of recovery.

The silicon tetrafluoride which is used in the process of the inventioncan be obtained from any siliconand fluorine-containing raw materials byany of the processes known to those skilled in the art. A verysatisfactory source is a high-silica fluorspar containing calciumfluoride and more than 3% silica, either as calcium fluosilicate orsilicon dioxide. Another source is the sodium fluosilirate obtained as abyproduct in treating fluorine-containing phosphates. The thermaldecomposition of fluosilicates in general also can be used as a source.The silicon-containing fluorine compounds are reacted with an inorganicacid such which preferably is relatively nonvolatile, such as sulfuricacid or phosphoric acid, by known means, producing a mixture of hydrogenfluoride and silicon tetrafluoride. Such processes are, for example,described in Thorpes Dictionary of Applied Chemistry, 4th Edition,volume 5, page 278 et seq.

The gaseous silicon tetrafluoride, mixed with any hydrogen fluoride alsoobtained, is decomposed either by hydrolysis or by combustion in aflame. Both reactions are carried out in the vapor phase. In thisreaction a finely-divided silicon dioxide mixed in suspension in gaseoushydrogen fluoride land unreacted silicon tetrafluoride is obtained as aproduct.

A very desirable process for the combustion of the silicon tetrafluorideis described in US. application for Letters .Patent Serial No. 437,383,filed June 17, 1954, now U.S. Patent 2,819,151, issued January 7, 1958,of which this application is a continuation-in-part. By this processsilicon fluoride, a combustible gas and oxygen are reacted in a flame,and it is possible to obtain silica having a particle size within therange of from 5 to 50 mg.

The reaction probably proceeds in certain zones of the flame. Theintensity of the flame in these reaction zones is of primary importancein determining the particle size of the silica, and for this reason canbe regarded as equivalent to the intensity of the reaction. However, theflame intensity in the reaction zones is diflicult to measure except interms of the heat liberated by the reaction, which of course is directlyproportional to the heat liberated by the flame, and therefore for thepurposes of the invention, the intensity of the reaction is measured bythe intensity of the flame itself.

The intensity of the flame can be measured in terms 0f the amount ofheat liberated per unit volume and per unit time, i.e.,

B.t.u.

Cu. ft. min.

These quantities are measured in British thermal units, abbreviatedB.t.u., cubic feet and minutes. For convenience of representation,reciprocal B.t.u. units are used, i.e.,

or B.t.u., and the terms reciprocal B.t.u." and B.t.uf will beunderstood to refer to the volume of the flame in cu. ft. for eachB.t.u. evolved per minute in the flame.

Accordingly, in the process the flame intensity is maintained within therange from about 111x10 to about 1.3 10- B.t.u.- This range liesessentially below the intensity of a normal flame in which siliconfluorides are subjected to the reaction in accordance with the saiddisclosure. These intensity limits are critical, inasmuch as at flameintensities both above and below these limits the silica particle sizeagain increases. All of the disclosure of application Serial No. 437,383is accordingly incorporated herein by reference, inasmuch as amorphoussilicon dioxide of this particle size is an especially valuablebyproduct.

However, it will be understood that the particular method of decomposingthe silicon tetratluoride is in no way critical to the production ofhydrogen fluoride, the primary product of the invention. Other methodsof combustion of the silicon tetrafluoride are described in theliterature, for example, in British Patents Nos. 258,313, datedSeptember 15, 1926, and 438,782, dated November 22, 1935. A method forthe hydrolysis of silicon tetrafluoride in the vapor phase is describedin the Broughton Patent No. 2,535,036, dated December 26, 1950.

The hydrolysis by means of water vapor may also be carried out attemperatures above 500 C. in the presence of a gas such as air whichreduces the concentration of silicon tetrafluoride.

Further information with regard to the vapor phase hydrolysis reactionwill be found in Baur, Z. physik. Chem. 48, 483-503 (1904), and Lenfestyet al., Ind. Eng. Chem. 44, 1448-1450 (1.952). These processes willproduce silica particles in the range of from 100 to 400 m as contrastedto silica having the mean particle size of 50 m or less when prepared inaccordance with the process of Serial No. 437,383.

The reaction mixture obtained by any of these processes containsfinely-divided silicon dioxide suspended in a hydrogenfluoride-containing gas with unreacted silicon tetrafluoride. Thesilicon dioxide is separated by any suitable means. One or moreseries-connected cyclones can be used. Filtration also may be feasible.

It is then necessary to recover the hydrogen fluoride and unreactedsilicon tetrafluoride. This can be done in a number of alternative ways,all involving absorption of the gases upon solid sodium fluoride.

The waste gases may be passed through an absorption tower containingsolid sodium fluoride at a temperature above the dew point but belowabout 200 C. Under these conditions the hydrogen fluoride and silicontetrafluoride are absorbed, whereas other gases which may be present inthe mixture, such as carbon dioxide and water vapor, are untouched. Bythis method a mixture of sodium fluosilicate and sodium bifluoride isobtained.

Another method is to cool the waste gases to below 100 (1., preferablyto a temperature within the range from about 20 C. to about C., wherebya condensate is obtained which contains a mixture of hydrofluoric acidand hydrofluosilicic acid dissolved in the condensed water vapor presentin the gases. To this condensate there is added, with agitation, solidsodium fluoride. If the amount of sodium fluoride added is sufficient toreact with all of these materials present, a mixture of sodiumhifluoride and sodium fluosilicate is obtained as a precipitate. Thismay easily be separated from the water by decantation or filtration.

inert gases passed through the different tubes, it was possible to varythe intensity of the flame, as given in the table below. The mixture ofhydrocarbons used in Examples 5-7 had the following composition: 2% CHHowever, if it is desired to obtain relatively pure so- 5 11% C H 51% C11 and 25% C H giving a neat dium bifluoride and sodium fluosilicate, itis possible to combustion heat of 2480 B.t.u. per cu. ft. carry out thereaction with sodium fluoride in two steps. The silicon dioxide formedwas separated from the In the first step, suflicient sodium fluoride isadded to recombustion gases by means of a ceramic filter. The prodactwith the hydrofluosilicic acid present, forming a preucts obtained inall the examples were white, amorphous, ci-pitate of sodiumfluosilicate. This precipitate is re- 10 voluminous powders which byexamination under the elecmoved. Thereafter additional sodium fluorideis added, tron microscope proved to be made up of amorphous, in anamount sufficient to precipitate the hydrofluoric spherical particleshaving mean diameters ranging from acid present in the form of sodiumbifluoride. This, like- 9 to 91 ru of which a major proportion wereassociated wise. is removed. as small aggregates.

The sodium fluosilicate and sodium bifluoride thus ob- The eflluentgases from the combustion chamber, comtained are decomposed by heat.Sodium bifluoride is deosed of unreacted oxygen, inert gases, silicontetracomposed to sodium fluoride, liberating hydrogen fluoride, fluorideand hydrogen fluoride, were passed through an at e pe tu es above a t350 C- dium fiH il Cat absorption tower containing solid sodium fluorideheld is decomposed to sodium fluoride and silicon tetrafluoride t atemperature of 175 C. In this chamber substanat temperatures above 550C., ranging up to 700 C. tially all of the hydrogen fluoride and silicontetrafluoride Thus, it is Possible to deCOmPOSB thesfi Salts p ra lywere absorbed, as shown by analysis of the gases emergeven when they arepresent in admixture. The mixture of i fr h absorption towgn Ab i wassodium bifluoride and sodium fluosilicate, for instance, ti d til {hesgdium fl id i th tower was can be heated in an electric furnace or in agas-heated h t d, a shown by the appearance of a substantial rotaryfumacfi to about decomposing the Sodiquantity of hydrogen fluoride andsilicon tetrafluoride in bifluoride- Tilt: hydmgen fluoride is Tfiwveredy the eifluent from the tower, after which the mixture of y desiredmeans, Such as y Condensation. N xt, the sodium bifluoride and sodiumfluosilicate was removed mixture is heated to 550 to and air is Passedfrom the tower and transferred to an electric furnace. through thefurnace, whereupon the sodium fluosilicate is There it was h t d t a terat r f about 350 C. decomposed, and silicon tetrafluoride is liberated.This 3D and mm at this temperature f two hgups At this togfithel withthe air is cycled to the combustion of perature the sodium bifluoridewas decomposed, and the hydrolysis apparatus for reuse, and furtherrecovery of hydrogen fluoride which escaped in gaseous form was hydrogenrecovered by condensation in a cooler held at 0 C.

It is apparent that the decomposition of the sodium bi- Ag h d fl id h de d t l th fluoride and sodium fluosilicate can 135 carried out COntempratu e in the furnacg was increased to 650 C. and linufillsly in twoseries'connected rotary furnaces, the air was passed through. Silicontetrafluoride liberated first held at about and the Second at a ptogether with the air was conducted to the combustion ture within therange from about 550 to 700 C. h b f r fans;

The sodium fluoride obtained as a residual product in 40 The yield ofhydrogen fluoride based upon the weight these processes is recovered andreused for the production of silicon tetrafluoride used is given in thetable. It is of sodium bifluoride and sodium fluosilicate from theapparent that excellent yields are obtainable.

Table 1 Amount of Heat Flame Intensity Particle Percent, Yield ExampleCombust- Combustevolved in Flame size or No. thin gas ihle gas theflame, Volume silicon suit/min. Btu/min. cu. ft. B.t.u.l0u. Reciprocaldioxide Slog HI ttjmin. Btur m t 0.710 1% 0.00513 s.s0 10 2. 0a 10- s0is X as 0.710 105 000250 7 70 10 1.s0 10- 5s 01 00 0. 710 10s 0.00110117x10 0.s7 10- 9 s7 0? 0.710 105 0. 00025 700x10 013x10 i2 92 0s 0. 715430 0.0111 3 02x10 2. ssxitr- 91 10 90 0.115 430 0.0055 7 51x10 1. ss10- 02 i9 01 0.115 430 0. 0029 150x10 007x10 12 83 9s 1 One cycle. 2Repeated cycles, average. waste gases obtained in the combustion orhydrolsyis EXAMPLE 8 step 60 Fluorspar containing 12% silica and sand in20% The following examples represent 1n th P of excess based on thetotal SiO were reacted with a 10% the inventor the best embodunents ofhis invention: excess of 70% sulfuric acid Solution at 900 Cu pwducingEXAMPLES 1 to 7 3.2 kg. per hour silicon tetrafluoride vapor, which wasIn these examples the silicon tetrafluoride was genheated to andblenfied with a flowing stream of erated by heating a mixture of silica(sand) and calcium equal par f Steam and held at l Same p fluoridetogether with sulfuric acid. The silicon tetraamount P used was 30% F 0ffluoride so obtained was mixed with the combustible gas h 511mmtetmfluonde usfidg The Rachel lmme listed in the table below and withair or a 40% nitrol f Finer Contact as evidenced by a whlte Paw!eroxygen mixture and the mixture burned using a visible in the gascurrent. The mixture was conducted jet burner fitted with threeconcentric tubes. The mixthl'ough a brick'nned reaction chamber held atture of silicon tetrafluoride, combustible gas, oxygen and and then thefinely-divided amotphous Silica formed was inert gases was passedthrough the intermediate tube, and separated in two Saks-Connectedcyclones- The Waste air or (in Examples 4 and 7) 60% oxygen and 40%gases were cooled to about 20 C. in a condenser to an nitrogen mixturewas passed through the outermost and aqueous condensate including insolution hydrofluoric innermost tubes. By varying the amount of oxygenand and hydrofluosilicic acids. To this condensate there was addedsufficient solid sodium fluoride to react with the hydrofiuosilicic acidpresent and form sodium fluosilicate as a precipitate. This precipitatewas filtered ofl. There was then added another portion of sodiumbifiuoride in an amount stoichiometrically equivalent to thehydrofluoric acid present, whereupon sodium bifiuoride precipitated andwas removed by filtration.

The sodium bifiuoride was dried and decomposed by heating in agas-heated rotary furnace to about 350 C. The hydrogen fluoride whichescaped was recovered by condensation. A 95% yield of hydrogen fluoridewas obtained, based on the amount of silicon tetrafluoride used.

The sodium fluosilicate was dried and decomposed by heating in anelectric furnace at 600 C., passing air through the furnace during thedecomposition. The silicon tetrafluoride which was liberated wasrecycled for reuse in the hydrolysis chamber.

The sodium fluoride remaining was recovered and reused to form sodiumbifiuoride and sodium fluosilicate, respectively.

EXAMPLE 9 A continuous stream of 12.1 kg. per hour silicon tetrafluoridewas made by reacting calcium fluoride, silicon dioxide and sulfuric acidat 90 C. The gas was mixed with another gas stream containing 6.3 kg.silicon tetrafluoride and 9.6 kg. air per hour and the resulting gasstream was conducted to a burner having three concentric tubes. In theburner the silicon tetrafluoride was mixed in the intermediate tube witha continuous flow of 27.5 kg. per hour mixed hydrocarbons of the samecomposition as stated in Examples to 7. A further 540 kg. per hour airwas introduced into the burner, through the outermost and innermosttubes, as in Examples 1 to 7, and the mixture was burned to form fineparticle silica and hydrogen fluoride. The silica was separated in twoseries-connected cyclones, and the exit gases cooled to 15 C.continuously in a cooling tower. The condensate obtained was mixed withan equivalent amount of solid sodium fluoride and the precipitateobtained was filtered off and dried in a gas-heated rotary dryer at 100C. The mixture of sodium bifiuoride and sodium fluosilicate wascontinuously heated to 350 C. in a gas-heated rotary furnace. At thistemperature the sodium bifiuoride was decomposed, forming sodiumfluoride and hydrogen fluoride. The hydrogen fluoride was condensed in acooler at 0 C. and taken up in steel containers. 8.82 kg. per hourhydrogen fluoride was obtained, 95% of theoretical.

The mixture of sodium fluoride and was decomposed at 600 nace throughwhich 9.6

sodium fluosilicate C. continuously in a rotary furkg. per hour airpreheated to 600 C. was blown. From this furnace 6.3 kg. per hoursilicon tetrafluoride was obtained, together with the air. This mixtureof silicon tetrafluoride and air was recycled and introduced into theflame as described above.

The sodium fluoride obtained by decomposition of sodium bifiuoride andsiliconfluoride also was recovered and recycled for use in the process.

The hydrogen fluoride which is obtained by the process of the inventionis capable of use in conventional ways. The process is capable ofproducing an anhydrous hydrogen fluoride in a very pure state.

The silica particles produced by the process are amorphous, that is,they are noncrystalline in character. They can be agglomerated to formlarger particles if desired.

The finely-divided, amorphous silica prepared by the process of theinvention is particularly adapted for use as a reinforcing agent inrubber compounding. However, it may also be employed for other purposes,such as a pigment, a filler for synthetic resins and a reinforcing agentfor synthetic polymers, such as silicone resins, which are, basically,modified silicic oxide polymers.

I claim:

1. The process for the production of hydrogen fluoride and silica fromsilicon tetrafluoride prepared from siliconand fluorine-containingmaterial in which the silicon content expressed as silica is greaterthan 3% which comprises the steps of reacting the silicon tetrafluoridewith water in the vapor phase to form a gaseous suspension consistingessentially of hydrogen fluoride, unreacted silicon tetrafluoride andsilica, removing the silica from such gases to obtain a gaseous mixtureconsisting essentially of hydrogen fluoride and unreacted silicontetrafluoride, reacting the hydrogen fluoride and said unreacted silicontetrafluoride with sodium fluoride to form sodium bifiuoride and sodiumfluosilicate, heating the sodium bifiuoride to a temperature todecompose it without substantial decomposition of sodium fluosilicate torecover a gas having a higher concentration of hydrogen fluoride thansaid gaseous mixture, and heating the sodium fluosilicate at atemperature in excess of about 550 C. to decompose it selectively torecover a gas having a higher concentration of silicon tetrafluoridethan said gaseous mixture, and returning said last mentioned silicontetrafluoride to said vapor phase water treating step.

2. A process in accordance with claim 1, in which the silicontetrafluoride is hydrolyzed to form silica in the flame of a combustiblegas.

3. A process in accordance with claim 1, in which the silicontetrafluoride is hydrolyzed with steam.

4. A process in accordance with claim 1 in which the gaseous mixture iscondensed to form an aqueous solution of hydrogen fluoride andfluosilicic acid, and this solution is treated with sodium fluoride.

5. A process in accordance with claim 4, in which the amount of sodiumfluoride is added in two stages, the amount added at first beingstoichiometrically equivalent to that required to react withsubstantially all of the fiuosilicic acid present to form sodiumfluosilicate, precipitating the sodium fluosilicate and separating itfrom the solution, and then adding suflicient sodium fluoridestoichiometrically equivalent to the hydrogen fluoride in solution,precipitating sodium bifiuoride and separating it from the solution.

6. A process in accordance with claim 1, in which the silicontetrafluoride is reacted in the gas phase with a combustible gas and afree-oxygen containing gas in a flame zone liberating from 0.l l0* to1.3)(10 B.t.u.- to form silica and hydrogen fluoride.

7. A process in accordance with claim 1, in which the hydrogen fluorideand silicon tetrafluoride in the vapor phase are absorbed upon solidsodium fluoride.

8. A process in accordance with claim 1, in which the, silicontetrafluoride is obtained by the reaction of fluorspar with an inorganicacid.

9. A process in accordance with claim 1, in which the silicontetrafluoride is obtained by the reaction of a fluosilicate with aninorganic acid.

10. A process for the production of hydrogen fluoride and silica fromsilicon tetrafluoride prepared from siliconand fluorine-containingmaterial in which the silicon content expressed as silica is greaterthan 3% which comprises the steps of reacting the silicon tetrafluoridewith water in the vapor phase to form hydrogen fluoride and silica andleave some unreacted silicon tetrafluoride in the waste gas, removingthe silica from the waste gas, reacting the hydrogen fluoride andunreacted silicon tetrafluoride in the vapor phase with sodium fluorideto form a mixture of sodium bifiuoride and sodium fluosilicate, heatingsaid mixture to a temperature in excess of about 350 C. but belowtemperatures at which a significant amount of decomposition of sodiumfluosilicate takes place, to decompose selectively said sodiumbifiuoride and recover hydrogen fluoride, and thereafter heating thesodium fluosilicate at a temperature in excess of about 550 C. todecompose it and recover silicon tetrafluoride.

11. The process for the separate production and recovery of hydrogenfluoride and silica starting from silicon tetrafluoride prepared fromsilicon and fluorine-containing material in which the silicon contentexpressed as silica is greater than 3%, which comprises reacting saidsilicon tetrafluoride with water in the vapor phase to form a gaseoussuspension consisting essentially of hydrogen fluoride, unreactedsilicon tetrafluoride and silica, separating the silica therefrom,recovering a gaseous mixture consisting essentially of hydrogen fluorideand silicon tetrafluoride, reacting the hydrogen fluoride with sodiumfluoride to form sodium bifluoride, reacting the silicon tetrafluoridewith sodium fluoride to form sodium fluosilicate, heating the sodiumbifluoride at a temperature to decompose it to sodium fluoride andhydrogen fluoride, while minimizing decomposition of sodiumfluosilicate, separating hydrogen fluoride thus formed, heating thesodium fluosilicate at a temperature in excess of about 550 C. todecompose it to recover sodium fluoride and silicon tetrafiuoride,recycling silicon tetrafluoride thus formed for reaction with water inthe vapor phase, and recycling sodium fluoride from both decompositionstages for reaction with hydrogen fluoride and silicon tetrafluoride,and recovering hydrogen fluoride from the system, thereby converting thefluorine content of the silicon tetrafluoride into hydrogen fluoride.

12. The process for the production of hydrogen fluoride and silica fromsilicon tetrafluoride prepared from siliconand fluorine-containingmaterial in which the silicon content expressed as silica is greaterthan 3% which comprises the steps of reacting the silicon tetrafluoridewith water in the vapor phase to form a gaseous suspension consistingessentially of hydrogen fluoride,

unreacted silicon tetrafluoride and silica, removing the silica fromsuch gases to obtain a gaseous mixture consisting essentially ofhydrogen fluoride and unreacted silicon tetrafluoride, reacting thehydrogen fluoride and said unreacted silicon tetrafluoride with sodiumfluoride to form sodium bifluoride and sodium fluosiiicate, heating thesodium bifluoride to a temperature suflicient to decompose itselectively to recover sodium fluoride and a gas having a higherconcentration of hydrogen fluoride than said gaseous mixture, andheating the sodium fluosilicate at a temperature in excess of about 550C. to decompose it selectively to recover sodium fluoride and a gashaving a higher concentration of silicon tetrafluoride than said gaseousmixture, returning said last mentioned silicon tetrafluoride to saidvapor phase water treating zone, and recycling said recovered sodiumfluoride for reaction with the gaseous mixture of hydrogen fluoride andunreacted silicon tetrafluoride.

References Cited in the file of this patent UNITED STATES PATENTS1,70l,225 Buchner Feb. 5, 1929 2,535,036 Broughtcn Dec. 26, 19502,588,786 Winter Mar. 11, 1952 2,819,151 Flemmert Jan. 7, 1958 2,886,414Secord May 12, 1959 OTHER REFERENCES Melior: A Comprehensive Treatise onInorganic and Theoretical Chemistry, Longmans, Green & Co., New York,N.Y., vol. 6, 1925, pages 946 and 948.

1. THE PROCESS FOR THE PRODUCTION OF HYDROGEN FLUORIDE AND SILICA FROMSILICON TETRAFLUORIDE PREPARED FROM SILICONAND FLUORINE-CONTAININGMATERIAL IN WHICH THE SILICON CONTENT EXPRESSED AS SILICA IS GREATERTHAN 3% WHICH COMPRISES THE STEPS OF REACTING THE SILICON TETRAFLUORIDEWITH WATER IN THE VAPOR PHASE TO FORM A GASEOUS SUSPENSION CONSISTINGESSENTIALLY OF HYDROGEN FLUORIDE, UNREACTED SILICON TETRAFLUORIDE ANDSILICA, REMOVING THE SILICA FROM SUCH GASES TO OBTAIN A GASEOUS MIXTURECONSISTING ESSENTIALLY OF HYDROGEN FLUORIDE AND UNREACTED SILICONTETRAFLUORIDE, REACTING THE HYDROGEN FLUORIDE AND SAID UNREACTUMBIFLUORIDE AND SODIUM FLUOSILICATE, HEATING THE SODIUM D SILICONTETRAFLUORIDE WITH SODIUM FLUORIDE TO FORM SODIBIFLUORIDE TO ATEMPERATURE TO DECOMPOSE IT WITHOUT SUBSTANTIAL DECOMPOSITION OF SODIUMFLUOSILICATE TO RECOVER A GAS HAVING A HIGHER CONCENTRATION OF HYDROGENFLUORIDE THAN SAID GASEOUS MIXTURE, AND HEATING THE SODIUM FLUOSILICATEAT A TEMPERATURE IN EXCESS OF ABOUT 550* C. TO DECOMPOSE IT SELECTIVELYTO RECOVER A GAS HAVING A HIGHER CONCENTRATION OF SILICON TETRAFLUORIDETHAN SAID GASEOUS MIXTURE, AND RETURNING SAID LAST MENTIONED SILICONTETRAFLUORIDE TO SAID VAPOR PHASE WATER TREATING STEP.